Computer setting

[clarion]

Butterfly has a new Dahon Speed. 

She has a Cateye computer to fit it.

The table of sizes doesn't mention the size of tyres it's fitted with

They are:

40-406 20x1.5

Any idea of the setting required?

[Thor]

See here

It depends what the computer is asking for - but frequently it's the circumference of the tyre in mm.  You can calculate this yourself and Sheldon will tell you how 

[clarion]

It's not even on his chart, nor are there values I can interpolate between.

I'll have to do it the old way, by marking, rolling & measuring.

[Jaded]

I don't bother with measuring tyres. I tun my bike over a local route that I know the distance of. (I have a triangle that is exactly 10 miles, for example) I then check what the bike computer has recorded, and find the ratio to the 10 miles. I then apply the ratio to the circumference that was in the computer.